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Can not return a com object from within c#
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Umed Kurdistan via .Net Guru (VIP)
OPC (OLE for Process Control) is a protocol as an interface between applications and industry hardware drivers. It defines two main components (COM) registered in opcserver category : OpcServer & OpcGroup.
I have written two coresponing classes as follows that implement the required Interfaces as defined by OPC.
-----------------------
[Guid( "926EFA52-C020-4259-B879-203512901EAF" )]
[ProgId("FAHM OPC Server 1")]
[ComVisible( true )]
public class OpcServer: IOPCServer, IOPCCommon, IOPCItemProperties, IOPCBrowseServerAddressSpace
{
//Implementd inteface methods
}

[Guid( "075A7104-076F-481f-8CCF-11BA9AE375EB" )]
[ProgId("FAHM OPC Group 1")]
[ClassInterface(ClassInterfaceType.None)]
public class OpcGroup: IOPCItemMgt, IOPCAsyncIO2, IOPCSyncIO, IOPCGroupStateMgt
{
//Implementd inteface methods
}

I can in a not managed programming language such as delphi or c++ create an instance of them and call all methods of both of them and QueryInterface them to all implemented Interfaces and see they are working by showing just a messagebox for test.
The problem is that the IOPCServer Interface defines a method AddGroup as follows that returns an instance of OPCGroup and has following signature in c++ and c#:
c++:
virtual HRESULT STDMETHODCALLTYPE AddGroup(
/* [string][in] */ LPCWSTR szName,
/* [in] */ BOOL bActive,
/* [in] */ DWORD dwRequestedUpdateRate,
/* [in] */ OPCHANDLE hClientGroup,
/* [in][unique] */ LONG __RPC_FAR *pTimeBias,
/* [in][unique] */ FLOAT __RPC_FAR *pPercentDeadband,
/* [in] */ DWORD dwLCID,
/* [out] */ OPCHANDLE __RPC_FAR *phServerGroup,
/* [out] */ DWORD __RPC_FAR *pRevisedUpdateRate,
/* [in] */ REFIID riid,
/* [iid_is][out] */ LPUNKNOWN __RPC_FAR *ppUnk) = 0;
c# :
void AddGroup(
    [In, MarshalAs(UnmanagedType.LPWStr)] string szName,
    [In, MarshalAs(UnmanagedType.Bool)] bool bActive,
    [In] int dwRequestedUpdateRate,
    [In] int hClientGroup,
    [In] ref int pTimeBias,
    [In] ref float pPercentDeadband,
    [In] int dwLCID,
    [Out] out int phServerGroup,
    [Out] out int pRevisedUpdateRate,
    [In] ref Guid riid,
    [Out, MarshalAs(UnmanagedType.IUnknown)] out object ppUnk);
When in the unmanged code call this method of IOPCServer, it seems to be ok, but the returned object in ppUnk that supposed to be a IOPCItemMgt fails to execute any of its methods and raises an memory exception.
I tried following implementation for OPCSerevr.AddGroup but all of them have the same wrong result:
------------------------------------------
impl. 1:
Type typeofGroup = Type.GetTypeFromProgID( "FAHM OPC Group 1" );
if( typeofGroup == null )
throw new Exception("OPC Server Not Found");
object grp = Activator.CreateInstance( typeofGroup );
IntPtr pRes = IntPtr.Zero;
IntPtr pUnk = Marshal.GetIUnknownForObject(obj);
Marshal.QueryInterface(pUnk, ref riid, out pRes);
Marshal.Release(pUnk);
Marshal.Release(pUnk);
if (!pRes.Equals(IntPtr.Zero))
ppUnk = pRes;
else
Marshal.ThrowExceptionForHR(HResult.E_NOINTERFACE);

------------------------------------------
impl. 2:
object grp = new OpcGroup();
ppUnk = IntPtr.Zero;
ppUnk = Marshal.GetComInterfaceForObject(grp, typeof(IOPCItemMgt));

------------------------------------------
impl. 3:
object grp = new OpcGroup();
ppUnk = grp;
------------------------------------------
impl. 4:
object grp = new OpcGroup();
ppUnk = (IOPCItemMgt)grp;

And for each of mentioned lines in above implementaions I surrounded it with a try-catch block and showed messageboxes after each successfull or failed block, and it seems that none of them fails, but I don't know why the returned IOPCItemMgt is incorrect.

Can anyone help me?
Special Regards
Umed Kurdistan

--------------------------------
From: Umed Kurdistan

-----------------------
Posted by a user from .Net Guru (http://www.dot-net-guru.com/)

<Id>1Z8SEFKaLUqsPcQgEDjeeA==</Id>
Reply to this message...
 
    
Robert Jordan
Hi Umed,

[Original message clipped]

I encountered the same problem some time ago. The solution
was to rename the whole inteferface and made the certain method
return the specific interface:

void AddGroup(
...
[Out] out IOPCItemMgt ppUnk
);

I don't have the sources of the COM-client that was consuming
my C#-COM-object, thus I was not able (or willing) to debug
the problem, but it appears to be a client's problem.

bye
Rob
Reply to this message...
 
    
Robert Jordan
Robert Jordan wrote:

[Original message clipped]

I took a look at the revision history of my code:
I was testing 2 variants. Both were working. The another one was:

void AddGroup(
...
[Out] out IntPtr ppUnk
);

implementation:

void AddGroup(....)
{
object grp = new OpcGroup();
ppUnk = Marshal.GetComInterfaceForObject(grp, typeof(IOPCItemMgt));
}

bye
Rob
Reply to this message...
 
 
System.Activator
System.Exception
System.IntPtr
System.Runtime.InteropServices.ClassInterfaceType
System.Runtime.InteropServices.Marshal
System.Runtime.InteropServices.UnmanagedType
System.Type




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